Photoluminescence spectra of various samples made of polymethyl methacrylate (PMMA) induced by 222-nm-wavelength radiation of a KrCl excilamp with a bandwidth of ~ 2 nm and a narrow-band KrCl laser, as well as transmission spectra of these samples, have been investigated. It was found that the PMMA samples, according to their transmission spectra, can be divided into three characteristic groups with different short-wavelength transmission edges, which affect the photoluminescence spectra. It has been shown that the photoluminescence spectra of PMMA samples in the ultraviolet and visible regions depend on the power density of the exciting radiation.
The development of nanosecond discharge in 2-mm point-to-plane or point-to-point gaps filled with distilled water has been studied with a fast-framing device. Positive nanosecond voltage pulses with an amplitude of 25 kV (50 kV in idle mode) were applied via a 75-Ohm transmission line to the gap. The pointed electrodes were made of pieces of sewing needles. Time-resolved images of discharge were taken with a four-channel intensified charge-coupled device (ICCD) camera with simultaneous recording voltage and current waveforms with an 8-GHz oscilloscope. The ICCD camera takes three consecutive and one time-integrated image. It was observed that one or several streamers appeared on the surface of the positive needle tip. They branched forming a semblance of a crown that occupied a large region near the needle. When reflected pulses with negative polarity arrived on the pointed electrode, streamers did not grow. When the next positive pulse arrived on the pointed electrode, the streamers began to grow again, while actively branching. In the point-to-point gap, streamers developed from both needles. The collision of streamers was observed. A bright light flash accompanies the collision. A long afterglow was observed in the place of streamer collision.
The results of experimental studies of various generation modes of runaway electron beams (RAEBs) are presented. The main attention is paid to the mode when the RAEB with the highest amplitude are generated in air at atmospheric pressure. Other modes are discussed too: the most implemented and often mentioned by other authors generation mode as well as the mode when RAEB is generated after breakdown.
Using an electrode with a ceramic coating, a new method for the formation of an apokampic discharge in the pressure range of 30-150 Torr has been obtained. It was found that the average plasma bullets velocity propagated from such an electrode can reach 520 km/s. The results obtained are in good agreement with the simulation results obtained earlier in the framework of the streamer model.
Using emission spectra, the electronic, vibrational, rotational, gas temperatures and the reduced electric field strength at different distances along the propagation of the apokamp from the discharge channel at an air pressure of 150 Torr, a voltage of 7.6 kV, and a frequency of 37.5 kHz were determined. To determine the above values of plasma parameters, the methods of optical emission spectroscopy were used, after which the experimentally obtained values were compared with the values obtained in the course of modeling the emission spectra of the discharge plasma using a code based on the radiation-collisional plasma model. It is shown that the values of the electron temperature and reduced field strength increase abruptly at a height of ~ 12 mm above the discharge channel, marking the transition from the offshoot zone to the positive streamer zone. The gas temperature along the distribution of the apokamp decreases exponentially and at a distance of 75 mm reaches about 530° C (which is about 3 times less than at the base of the apokamp). The data obtained are in agreement with the streamer model of the plasma plume of the apokamp and allow us to hope for the creation of a plasma source based on an apokampic discharge with a moderate gas temperature at the end of the plasma plume.
VUV radiation of run-away electron preionized discharge (REP DD) in different gas mixtures is studied. Efficient lasing was achieved on ArF* (193 nm) and F2* (157 nm) molecules. Narrowband VUV radiation (near 147 nm), corresponding to the optical transition of a heteronuclear dimer ArXe*, was recorded from the REP DD plasma in (He)-Ar-Xe mixture. Amplifying properties of the discharge plasma related to this radiation were found.
Laser action in run-away electron preionized discharge (REP DD) was studied. Efficient laser emission was obtained in wide spectral range from IR to VUV. It was shown that ultimate efficiency of non-chain chemical lasers on HF (DF) molecules and N2 laser at 337.1 nm can be achieved in REP DD. New mode of N2 laser operation with 2 or 3 peaks in successive REP DD current oscillations was found. Efficient lasing on KrF* and XeF* excimer molecules with parameter close to laser parameters of lasers pumped by conventional transverse discharge were demonstrated for the first time. Laser action on F2* at 157 nm and rare gas fluorides under REP DD pumping was obtained for the first time, as well. The efficiency and pulse duration of VUV F2* laser under REP DD excitation are comparable with those obtained in transverse discharges with preionization. VUV emission of REP DD in binary and ternary Ar-Xe-(He) and Ar-Kr-(He) mixtures at wavelength close to 147 nm was measured. Possibility of VUV lasing in mixtures of rare gases is considered.
Main parameters of plasma formed during the pulse and pulse-periodic runaway electron preionized diffuse discharge (REP DD) in argon, nitrogen and air at high pressure were measured. An electron concentration in the plasma of pulse and pulse-periodic REP DD in the elevated pressure argon was determined. Average for pulse value of electron density in the argon plasma of pulse REP DD was ~ 3·1015 cm-3. Dynamics of electron density in the atmospheric-pressure plasma of the argon during the REP DD was determined. Measured average values of an electron concentration in the plasma of the pulse-periodic REP DD in atmospheric-pressure air and nitrogen were ~ 3·1014 and ~ 4·1014 cm-3, respectively. In addition, for the plasma formed during the pulse-periodic REP DD in atmospheric-pressure nitrogen and air average values of an electron temperature and reduced electric field, as well their dynamics were determined. Average value of an electron temperature during the pulse duration for nitrogen and air plasmas was ~ 2 eV. Dynamics of an electron temperature and reduced electric field strength was registered. Data on rotational and gas temperatures in the discharge plasma of atmospheric-pressure nitrogen formed in pulse (Tr ≈ 350 K, Tg ≈ 380 K) and pulse-periodic (Tr ≈ 750 K, Tg ≈ 820 K) modes were obtained. In addition, measured value of vibrational temperature in REP DD’s plasma formed in pulse mode in nitrogen at pressure of 1 bar was Tv ≈ 3000 K.
Breakdown of the gaps with a non-uniform electric field filled with nitrogen and air as well as with other gases under
high-voltage nanosecond pulses was investigated. It is shown that conditions of obtaining a diffuse discharge without a
source of additional ionization are extended at the voltage pulse duration decreasing. A volume discharge is formed due
to the gap pre-ionization by runaway electrons and X-ray quanta. At a negative polarity of the electrode with a small
radius of curvature, a volume (diffuse) discharge formation is determined by pre-ionization with runaway electrons
which are generated due to the electric field amplification near the cathode and in the gap. At a positive polarity of the
electrode with a small radius of curvature, the X-ray radiation, generated at the runaway electrons braking at the anode
and in the gap, is of great importance in a volume discharge formation. A runaway electrons preionized diffuse discharge
(REP DD) has two characteristic stages. In the first stage, the ionization wave overlaps the gap during a fraction of a
second. The discharge current is determined by the conductivity current in the dense plasma of the ionization wave and
the displacement current in the remaining part of the gap. The second stage of the discharge can be related to the
anomalous glow discharge with a high specific input power. During the second stage, the gap voltage decreases and the
cathode spots formed as a result of explosive electron emission can participate in the electron emission from the cathode.
At the increase of the voltage pulse duration and specific input power, the REP DD transforms into a spark discharge
form. A REP DD is easily realized in various gases and at different pressures; see [1] and references in [1]. At pressure
decrease was obtained the anode electrons beam current to rise (up to ~2 kA/cm2 in helium). At the REP DD, the anode
is influenced by the plasma of a dense nanosecond discharge with the specific input power up to hundreds of megawatt
per a cubic centimeter, by the electrons beam, shock wave and optical radiation from discharge plasma of various
spectral ranges, including UV and VUV. This allows forecasting the REP DD application for modification and cleaning
of metal and dielectric surfaces. The REP DD is promising as well for creation of the VUV-range excilamps with a high
radiation power in a pulse. REP DD was use for pumping different gas lasers.
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